Field of the Invention
The present invention relates to a method for economically manufacturing high-grade pellets for metallurgical use in a blast furnace from several grades of iron ores. The present method can produce high-strength green pellets and save grinding cost by preferentially grinding only ores having a W.I. value not higher than 20 KWH/T, which are relatively easy to grind, and mixing the thus ground ores with ores which are hard to grind in an agglomerating (lump-making) step, such as a pelletizing step. The green pellets manufactured by the present method are free from powderization or deformation which causes difficulties in the pellet manufacturing process.
In a process of manufacturing pellets, the strength of the green pellets is one of the most important factors which control the product grade and the productivity. For example, in the manufacturing of fired pellets by the grate-kiln process, when the strength of the green pellets is not enough, the green pellets are powderized before they are transferred and charged in a firing oven, thus hindering gas flow in a drying step or in a preheating step, thereby causing lowered productivity. Further, the powders brought into the firing step stick on the inside wall of the kiln to form a so-called ring thereon, which prevents the travelling of the materials through the kiln, thus causing failure in the kiln operation.
Also in the non-fired pellet manufacturing process, which has been looked upon with great interest as an effective non-pollution means, when the strength of the green pellets is not enough, they are powderized or deformed before they are transferred and charged in a curing means, thus lowering their rate of production. Further, the powderized pellets cause a strong adhesion among the pellets during the curing step, so that in the case where a curing vessel of a hopper type is used, it is impossible to discharge the pellets therefrom and in the case where a curing equipment of a yard type is used, it is difficult to discharge and crush the giant blocks of the pellets.
As the factors which have influence on the strength of green pellets, there are raw material factors, such as the particle size and form of the raw materials, and equipment or operation factors, such as types and amounts of binders used, the water content, types of mixing machines, as well as mixing conditions, types of pelletizing machines as well as pelletizing conditions. However, so far as the equipment or operation factors are constant, the raw material factors basically have a greater influence on the strength of green pellets.